This application is in response to the President's Brain Research through Advancing Innovative Neurotechnologies (BRAIN) initiative. A central goal of the BRAIN Initiative is to understand how electrical and chemical signals code information in neural circuits and give rise to sensations, thoughts, emotions and actions. Existing technologies are not sufficient to accomplish this goal and have to be significantly improved or novel tools should be introduced to analyze circuit-specific processes in the brain, leading to transformative advances in our understanding of the brain function and behavior. RFA-EY-15-001 seeks technology at the very earliest stage of development, which can assist recording and/or manipulating neural circuit activity in human and animal experiments. The specific goal of the proposed work is to introduce and validate a new voltage-sensitive upconverting photoacoustic imaging (VSUPAI) technique. It is based on voltage-sensitive dye (VSD) imaging, which exploits change of optical properties of dye associated with a cell membrane with variation of a membrane potential, allowing for real-time probing of the neuronal activity via non-invasive optical methods. VSDs have limited use in deep brain imaging, because they require excitation in the visible range. This proposal addresses the current limitation of VSD imaging through the convergence of photoacoustic tomography (PAT), biocompatible upconversion (UC) nanoparticles, and VSDs. In the proposed method, we exploit the voltage-sensitive change in dye absorbance to produce a change in the photoacoustic signal, as opposed to fluorescence-based probing with conventional VSDs. In our proposal, the PAT technique will involve NIR excitation and ultrasound detection, while UCNPs will serve as nanotransformers that convert skull penetrating NIR light to VIS light, which will be absorbed by the locally administered VSDs, allowing us to monitor changes in their absorption, induced by changes in action potentials, and, correspondingly, map the deeper brain neuronal activity.